My Sonoma 4-cylinder engine has developed a skip.

What is your question?

Driving home from work one day, the engine started to skip, losing power in the process. This skip got worse until I barely made it in the driveway. I did a little diagnosis and discovered that the spark plug for the third cylinder was a sooty black, the other plugs were a normal gray color. I replaced thecoil for that plug as well as the plug itself. No luck. Do you have any other suggestions?

How long has your 2003 GMC Sonoma had this problem?

Truck has been sitting for a year and a half.